From the journal "Cement-Lime-Calcium Sulfate" (No. 5/1984, pages 219-225) a method for the calcination of raw cement powder in a circulating vortex layer is known. This method makes use of a system that consists of a shaft-like reactor, a precipitation cyclone and a pressure closure between the cyclone and the reactor in the form of a syphon. In the reactor there is a strongly expanding vortex layer of a fine-grained solid substance with declining suspension density from the grate to the reactor head. The solid substance, which is separated from the gas in the reversing cyclone during the external circulation, moves through the syphon back into the reactor. A partial stream according to the entry amount is extracted and controlled by a measuring device.
In addition to this external circulation of a solid substance, an inner re-circulation is produced by the continuously changing formation of strands in the reactor, which, under the relatively low transportation speed of the solid substance, leads to high relative speeds and extremely intense mixing movements between the gas and the solid substance. This causes good heat and substance exchange, which enables a relatively fast reaction turnover with good constant temperature within the entire system of the circulating vortex layer.
The fuel necessary for the endothermal reaction may be added to the lower part of the calcinator as fuel gas, as fluid, or as a solid fuel, and may be burned directly in suspension without the formation of a flame. The necessary combustion air is added to the calcinator as primary combustion air through the vortex bottom and as secondary combustion air above the grate, and serves together with the exhaust from the combustion and from the de-acidifying reaction as the vortex medium.
Since in the known procedure with a circulating vortex layer the heating of the material is performed in the vortex bed, a relatively long heating period of the various particles of the material of up to 3 minutes is required.
Besides a relatively high use of energy, the interspersing of the entire system and therefore the capacity of the entire system to produce cement clinker is limited by the required heating period.